Steam condensers are used in the electric power industry to provide the heat rejection segment of their thermodynamic Rankine power cycle. To accomplish this, steam condensers are coupled to the exhaust of low pressure turbines so as to condense this exhausted steam to liquid and return it for reuse in the power cycle. The primary function of the steam condenser is to provide a low back-pressure at the turbine exhaust, typically between 1.0 and 6.0 inches Hg absolute. Maintaining a low back-pressure maximizes the power plant thermal efficiency.
The two primary types of steam condensers are water-cooled surface condensers and air-cooled condensers. Water-cooled surface condensers are the dominant technology in modern power plants. However, air-cooled steam condensers are being used more frequently in order to comply with strict environmental requirements.
Air-cooled steam condensers have been used since the 1930's. The primary technical challenges that exist today regarding such condensers are with respect to the approach used to efficiently drain the condensate and the manner of trapping and removing noncondensable gas (typically air which has leaked into the system) while minimizing the turbine back-pressure. These air-cooled steam condensers are typically arranged in an A-frame construction with a fan horizontally disposed at the base and separate condenser tube modules inclined thereabove through which air flows. The steam inlet to these condenser tube modules is located at the top or apex so that the vapor and any resulting condensate both flow concurrently downward within the module.
Each module of a typical air-cooled steam condenser is generally composed of four or so rows of tubes stacked therein. As air flows upward around these stacked rows, its temperature increases resulting in a corresponding decrease in temperature difference between such air and the steam inside the next tube row. This lower temperature difference for each successive tube row results in less vapor flow and condensation occurring with respect to that tube row. Since the condensate and steam flows are lower for each successive tube row, the two-phase flow pressure drop is also lower for each successive tube row.
For a simple condenser, all the tube rows discharge into a common lower header that is at a pressure equal to the highest (fourth or uppermost tube row) exit pressure. Consequently, steam and noncondensable gases in the common lower header enter the discharge ends of these first three tube rows. With steam vapor now entering both ends of a tube, noncondensable gases (air) become trapped therein. It is in these air pockets that the condensate freezes during cold weather. Also, these air pockets blanket the heat transfer surface area thereby reducing the condenser efficiency during hot weather. Noncondensable gases that do not become trapped are generally vented from the lower header with vacuum pumps or ejectors.
The ideal solution to the steam condenser problem is to maintain complete separation of fluid streams exiting each tube row. This is the fundamental approach of the steam condenser in U.S. Pat. No 4,129,180. Rather than a common lower header, this patent discloses a divided lower header with separate condensate and vent lines for each division of this lower header. With such independent lines, there is no pressure cross-over between the various tube rows. Condensate lines from each division of the lower header flow to a common drain pot that is configured with a water leg seal to balance the different pressures between them. The vent lines from each division of the lower header are also routed independently to individual vacuum pumps or ejectors for eventual discharge to the atmosphere. While this approach is ideal, manufacturing and erection costs are higher due to the complex system of drain lines and vent piping.
An alternate design that is commonly used is a two-stage condenser. In the main condenser, steam and condensate flow concurrently downward together through approximately two-thirds of the heat exchanger surface area required to condense the steam. Since the surface area of the main condenser is inadequate for complete condensation, excess steam from each of the rows is permitted to flow into the main condenser's common lower header. This prevents any backflow of steam and noncondensable gases back into these tube rows.
This excess steam then flows to a separate secondary condenser, typically a dephlegmator, that comprises the remainder (approximately one-third) of the total condenser surface area. Such a dephlegmator is constructed similar to the main condenser with each bundle thereof incorporating multiple (usually four or so) vertically stacked tube rows therein. In the dephlegmator, however, this excess steam and noncondensable gases flow upward in these tube rows from a lower common header before the gas therein is discharged. The resulting condensate from this upwardly flowing excess steam flow stream, however, flows by gravity counter-currently downward back to the common lower header supplying these tube rows. This common lower header thus both supplies these tube rows with the excess steam and noncondensable gases as well as collects the condensate from these tube rows.
Such a separate vent condenser (or dephlegmator) downstream the main condenser is designed to prevent the main condenser from trapping any noncondensable gases therein. However, should the vent condenser itself comprise multiple rows (which is normally the case), such a vent condenser will, in turn, experience backflow in its own lower tube rows. Thus, this problem of trapping noncondensable gases due to the backflow of steam into lower rows will merely be shifted to the vent condenser from the main condenser.
U.S. Pat. No. 4,177,859 discloses an air cooled steam condenser whose lower header is baffled. This lower header also incorporates a separate inspection well that collects the condensate from the first or lowermost row of tubes which fully condenses the steam flowing therethrough. This inspection well is used to check the temperature of the condensate from this first row of tubes. However, this patent does not disclose how to prevent freezing should the condensate in the inspection well approach freezing temperatures. Nor does this patent discuss the elimination of backflow into the tubes so as to avoid the accumulation of noncondensable gases.
Other alternate design solutions involve fixed orifices or flapper valves to equalize the pressure drop between tube rows. Still other designs may vary tube fin spacing, fin height, or fin length from row to row in an attempt to achieve a balanced steam pressure drop. Another novel solution, described in U.S. Pat. No. 4,513,813, arranges tubes horizontally with multiple passes. In this arrangement, the flow through each tube experiences a similar cooling potential and therefore has a similar condensation rate and pressure drop. However, all of these alternate solutions either perform well only at the steam condenser design operating condition and/or are not cost competitive.
An important design limitation for the integral vent condenser is the counter current flow limit steam vapor velocity. At this critical velocity, steam entering the vent condenser is at a sufficient velocity to force the counter flowing condensate (which flows by gravity) to flow upward or backup into the vent condenser thereby preventing it from draining. This liquid backflow now being trapped greatly increases the vent condenser pressure drop and thus reduces the efficiency of the air removal system as well as increases the turbine back pressure.
It is thus an object of this invention to provide an air cooled condenser having a lower cost of maintenance and construction than prior airflow condensers which are known. A further object of the invention is to substantially eliminate the accumulation of noncondensable gases in the various tube rows of the heat exchanger. Another object of this invention is to substantially eliminate freezing of condensate in the condensing tubes by stacking the vent condenser over the main condenser such that the two are incorporated or integrated into a single module rather than as separate but adjacent modules. Yet another object of this invention is to locate the vent condenser in a region where the air temperature will have been heated above the freezing point of water. An additional objective of the invention is to prevent noncondensable gas accumulation by having a constant flow of vapor out of all main condenser tube rows in order to purge them of any such gases on a continual basis. Yet another object of the invention is to provide a design for the inlet configuration of the dephlegmator so as to increase the counter current flow limit value thereby increasing the capacity and flow rate permitted for the heat exchanger.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.